Fueling environments normally have fuel storage tanks, typically located underground, from which liquid fuel (e.g., gasoline or diesel fuel) is pumped to dispensers. To comply with environmental laws, rules, and regulations, these storage tanks may be double-walled and associated with various inventory reconciliation systems. Typically, these inventory reconciliation systems comprise a magnetostrictive probe which extends into the tank and comprises one or more floats adapted to move vertically therealong. The floats have magnets which, in conjunction with a tank monitor or other suitable control system, facilitate determination of the level (and thus amount) of the product stored in the tank. In particular, a magnetostrictive probe usually comprises a fuel level float that is designed to float on the interface between fuel and vapor in the storage tank ullage. One example of a magnetostrictive probe may be the Mag Plus™ Leak Detection Probe, sold by Veeder-Root Company of 125 Powder Forest Drive, Simsbury, Conn. 06070, the assignee of the present application.
Additionally, water may enter fuel storage tanks in various circumstances. Because water is denser than liquid fuel, it typically resides in a layer at the bottom of the storage tank. Thus, magnetostrictive probes usually include a water level float to determine the level of water in the storage tank. Because of the distinct difference in densities between water and liquid fuel, water level floats are designed to float on the fuel-water interface.
The measurements from these floats are reported to the tank monitor so that the operator of the fueling environment may evaluate and reconcile fuel inventory and/or detect leaks, as is well understood. One example of a tank monitor may be the TLS 450 or the TLS-350R Monitoring Systems, also sold by Veeder-Root Company. Further information on the operation of magnetostrictive probes in fueling environments is provided in U.S. Pat. No. 7,454,969, entitled “Fuel Density Measuring Device, System, and Method Using Magnetostrictive Probe Buoyancy,” incorporated by reference herein in its entirety for all purposes.
However, modern fueling environments may store liquid fuels which are mixtures of gasoline and ethanol in various ratios, rather than “pure” gasoline. For example, E10 is a liquid fuel comprising 90% gasoline and 10% ethanol. Generally, it is known that gasoline containing ethanol will separate into an upper layer of gasoline and a lower layer of aqueous ethanol (also known as “phase separation”) if the water concentration in the fuel becomes too great. It is desirable to know when this “phase separation” occurs so that pumping of fuel from the storage tank can be suspended until corrective action is taken.
More specifically, as small amounts of water enter the storage tank containing a gasoline/ethanol mixture, the ethanol absorbs the water. As the amount of water increases, the ternary mixture becomes unstable and most of the ethanol and water precipitate out from the gasoline to form a phase separation layer below a layer of gasoline and some ethanol. The phase separation layer has a lower density than pure water but a slightly higher density than gasoline. Further, this phase separation layer will increase in density as the amount of water added to the tank increases. For example, a “low density” phase separation layer may have a density approximately equal to 780-805 kg/m3, a “medium density” layer may have a density approximately equal to 805-820 kg/m3, and a “high density” layer may have a density approximately equal to 820-920 kg/m3.
Traditional water level floats do not reliably detect phase separation. In particular, as noted above, water level floats are designed to float on the interface between water and gasoline. However, the aqueous ethanol layer caused by phase separation has a lower density than water, and thus the water level float may not be buoyant enough to float on this phase separation interface. Therefore, the inventory reconciliation system may not detect phase separation, and an unsuitable fuel or a phase separation mixture may be pumped to a dispenser and/or a customer's vehicle.
Moreover, design of a float that will float at the phase separation interface is problematic because many factors may affect the density of gasoline, including temperature and Reid vapor pressure. For example, where high density fuel from a cold refueling truck is added to the storage tank, the density of the fuel may be very close to that of a phase separation layer. In this case, a float designed to rise in the presence of low density phase separation may continue to rise past the phase separation interface through the high density fuel. Additionally, it is possible for a change in temperature alone to induce phase separation.